ABSTRACT
This paper reviews the history of offshore seismic works in the Ube coalfield,Japan.The underground structure of the offshore area was clearly established by the subsurface velocity distribution obtained from refraction surveys conducted during the period 1947-1953.The outlines of the basin and the extension of the coalfield were also found.Four maps are presented showing the results of this work.The results of drilling wells and coal mine galleries are in substantial agreement with the original interpretation of seismic data.
INTRODUCTION
The Ube coalfield is located around the city of Ube,north latitude 33°55′,east longitude 131°15′,near the western extremity of Honshu Island of Japan,as shown on the index map,Figure I.The area of the coalfield is ahout 75°square kilometers and the greater part of it is under the sea.
The area surveyed by seismic refraction method is about 150 square kilometers on the sea ranging in depth from several to about 20meters.Since 1947,offshore seismic exploration has been carried out successfully to make clear the geologic structure under the sea bottom,especially to find the southern extremity of the basin and the extensions of the coalfield.
Seismic survey lines amounted to 255kilometers in total at the end of 1953.
GEOLOGY
The Ube coalfield is composed mainly of beds of the Ube group of Eocene age.The group crops out east and north of the city of Ube and it also extends under the sea to the southwest of the city.It rests uncomfortably on the basement rocks which are granite,serpentine,and Mesozoic and Paleozoic rocks.It is covered by river terrace and alluvial deposits of Quaternary age on land,while under the sea it is almost wholly covered by the Pleistocene Kotozaki formation which is from 30 to 80meters in thickness and is composed mainly of impermeable mud.
The Ube group is divided into three formations which are angularly com‐fortable.Named in order from oldest to youngest they are the Higashimisome formation which is from 50 to 100meters in thickness,the Okinoyama coalbearing formation with about 80meters or more in thickness,and the Nagasawa formation,of variable thickness.
The Higashimisome formation is mainly composed of alternating beds of mud‐stone,sandstone,and conglomerate at exposures on the slopes of the Okinoyama and the Higashimisome mines which are driven in the submarine rocks near the center of the coalfield;on the other side of the coalfield,the formation is represented by the Koto-gawa conglomerate along the river Koto-gawa.
The Okinoyama coal-bearing formation consists of sandstone in the upper half and alternations of sandstone and mudstone in the lower half.Six workable coal seams are intercalated in the formation.Its outcrops are along the sea coast to the east of the city of Ube.It is most extensive under the sea bottom,overlapping the underlying Higashimisome formation,and it rests,in many cases,upon the basement rocks.The Nagasawa formation is composed mainly of an alternation of sandstone and mudstone.It is found only under the sea.
The Ube group is considered to have been deposited in the depressions of the basement rocks,and it occupies several basins isolated from each other.The largest basin is the one which develops around and to the southwest of the cities of Ube and Onoda.The greatest part of the Ube group is under the sea and there are several reefs of basement rocks in the basin.The dips of the strata under the sea are usually from 3 to 5 degrees.
There is a crushed and faulted zone about 7kilometers offshore from the city of Ube.This zone trends NW-SE and it has a width about 1000meters.It was found by the seismic method.
SEISMIC SURVEYS
The first offshore seismic work by refraction methods in the Ube coalfield was tried in the summer of 1939 by the Research Association of Geophysical Exploration to investigate the geologic structure under the sea bottom,but its results did not become available.The field work of this earlier survey was made by use of a single moving coil type detector enclosed in a water-tight brass container.It was placed on the sea bottom near the coast and a one trace seismic record was taken at a fixed observation point,while shooting of the charges was made successively at shot points on the sea-floor which were placed every 200meters along an exploration line.
The second offshore seismic survey in this area was conducted in 1947 by the Geological Survey of Japan.Since then,a six-trace spread has been used with four or five shot points having from about a 50 to 2000meter-distance from the end of the spread.The detector interval used was 100 or 130meters.Detectors located at both ends of a spread were set together with sound wave pick-up （hydrophone）.As particular attention has been paid to the improvement of both equipment and techniques employed for offshore seismic refraction surveys,much progress in the survey methods has been made.The seismic survey was successively conducted since 1947 by the Geological Survey of Japan in cooperation with the Ube Industrial Company.Since 1953 a nine-trace seismograph has been used,and seven traces are employed for seismic waves and two for sound waves.In these surveys three different methods of setting the detectors were tried.
（a） Setting the detectors on the sea floor.This method,conducted during the period 1947-1948,is similar to that used on land.Each waterproofed detector in the six-trace spread had a separate lead-in wire;after shooting the charges at a sea-floor shot point,each detector was separately moved to each location on the next spread of the exploration line.
（b） Floating the detector spread at or near the sea surface.This method was tried in 1949.The six detectors were connected at intervals of 100meters or more on a six-conductor cable trailing from the observation boat.Some of the detectors were fitted with sound wave pick-ups to check the distance between the shot-point and observation point.To maintain the detectors in the vertical positionin order to insure the best response,each detector was suspended from a float by a light elastic cable connecting the float and the top of the detector;this arrangement minimizes the swaying caused by the motion of the surface water.The bouyancy of the floats is adjusted to the point where they will just sustain the weight of the detector and its suspended weight.The floated cable remains straight while the boats are under way,but drift caused by currents or tide soon begins when the boat stops so that the shot must be recorded almost simultaneously with the stopping of the boat.Alternatively,the detector spread is set on bottom by detaching the detectors from the floats immediately after the boat stops;this socalled modified detector-floating method has the advantage of reducing to a minimum the drift and sway caused by surface currents and wave action.Shooting was tried both on the seafloor and midway between the surface and bottom.
（c） Mounting the detectors on sleds for dragging along the sea bottom.Since 1950 this method has been used.The sleds were made sufficiently large to insure that the detectors would be maintained upright.After the sled-mounted detector spread,the length of which is 700meters,is brought into the desired position on the exploration line,the shots are set near the shooting buoy and fired on receipt of a radio signal from the instrument boat.A schematic diagram of the method is shown in Figure 2.
The sled-mounted detector spread was found most suitable for the offshore conditions at Ube,where the sea bottom is comparatively flat.The spread and accuracy of the survey were considerably increased by using methods （b） and （c） and from three to six spreads could be observed in one day:the spread of the survey in 1953 reached 4 or 5 times that accomplished earlier by using method （a）.
The range between the shot point and detector at both ends of spread was obtained from the time between the instant when charge was fired and the arrival of the direct sound through the water at the hydrophone.This time is multiplied by the velocity of the sound in sea-water.The sound wave velocity in the sea off Ube coalfield was found to be 1472±1.2meters per second.
The underground structure of the offshore area at Ube coalfield was clearly established by the subsurface velocity distribution obtained from these surveys.The data assist the geological interpretation by :（I） differentiating Quaternary sediments from Tertiary rocks,（2） establishing the thickness of Quaternary sediments and Tertiary rocks,（3） determining the depth of the basement rocks.An example of the cross section as well as the time-distance curve of one survey line is shown in Figure 3.
On the basis of velocity,two types of areas can be distinguished.In one case the velocity of the second layer shows clearly in the time-distance curve.In the other case,the velocity of the second layer does not appear in the time-distance curve although it is known to exist;this is explained by the relation between the thickness and the velocity ratio of the first and second layers.
In the interpretation of the results the velocities,confirmed by data obtained from wells,have been classified according to the geological formations as follows:
The subsurface structures of this area are shown in Figures 4 to 6.Figures 4 and 5 exhibit contours of the top surface of the second （Tertiary） and the third velocity layer （basement） respectively.Figure 6 exhibits the thickness of the second velocity layer that comprises the coal-bearing Tertiary beds.As will be seen from these maps,the Quaternary sediments have a maximum thickness of 180meters and the Tertiary rocks a maximum of 230meters:the depth to the Tertiary and basement averages about 150 and 280meters respectively.The thickness of the Quaternary sediments appears to increase proportionately with that of the Tertiary beds.Anticlinal and synclinal structures are developed at several places.
The thickness of Quaternary and Tertiary beds is thin in the uplifted area of basement,and it is thick in the depressed area.The subsurface structure map of this area indicates a basin with the western,eastern,and south-eastern margins relatively uplifted in relation to the central and southern parts.The surface of the basement dips generally away from the shoreline at an angle of 2 or 3 degrees.
Local zones of low or high velocity are found in some places.The low velocity zone is the most faulted or depressed area in this region.It appears to be affected by extensions of the Ube-Misaki fault,the Central fault,the Ryuosan fault,and the Tsubuta fault.The faulted zone and low and high velocity area are shown in Figure 7.A fault zone about one kilometer wide,the Tsubuta fault,trends from the north-west to the south-east.Faulting was found to be especially strong in the central area,and the scale of faulting on the eastern side of the Tsubuta fault is rather weak.
From the velocity distribution the geologic information in relation to the fault or crushed zone,folding,uplifted or depressed areas,and conditions of the Quaternary beds can be ascertained.
The velocity distribution in this area is,therefore,most important in assisting the geological interpretation as well as coal mining.
The location of all of the wells recently drilled on the structure are shown on the map of Figure 7.The results obtained during the survey were checked after by data from these wells within about 10 percent error.The comparisons of these two data are given in the following Table:
The original interpretation of seismic data are in substantial agreement with the results of drilling wells.The survey results were also checked at several points in the farthest reaches of the coal mine galleries.
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ACKNOWLEDGEMENTS
The authors wish to express their sincere thanks to Mr.K.Okada,vice-president of the Ube Industries Co.,Ltd.,for permission to publish this paper.Thanks are also due to Mr.D.P.Carlton,Humble Oil and Refining Co.,and to Paul L.Lyons,Sinclair Oil and Gas Co.,for assistance in the preparation of this paper.